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Old Jun 16, 2012, 10:30 PM
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Can't get Apprentice back in the air after loop

I flew my Apprentice for hours today, with no issues up until my last flight. I did a loop successfully, but then after that I could not keep the plane in the air, it just dived no matter how much elevator I gave it. It ended up crashing into some high grass. Luckily, it appeared to be intact. I checked out the plane, and then tried hand launching the same way I always do and still had the same issue: I couldn't get it to pitch up enough to get enough altitude and I ended up crashing again. I had some damage, but took care of it and tried to fly it again this evening and still the problem persisted.

I checked the COG and it appears to be close to 3.25" behind the front edge of the wing as stated in the manual. The next thing I checked was the firewall/motor mount to make sure the prop was not pointing in the wrong direction--that seemed to check out as well. I did have to glue the firewall to the fuse after the first crash, and also I glued the motor mount a bit as it was bent a little out of shape. But, despite gluing, the parts appear to be fine and the prop is pointing in the same direction as always, at least as far as I can tell.

The elevators seem to be traveling in the right direction and appear to have enough deflection to pitch the nose up, but I still can't get the plane in the air. The ailerons are working OK, they do not appear to be out of alignment. This plane has been through a lot, but everything appears to be in order. The trim appears to be OK. Both the wing and the tail appear to have the same pitch they always had. No issues with the radio (I'm using a Turnigy 9x with the RMileC model). The rubber bands seem strong enough to hold the wing to the fuse. It doesn't look like any of the servos and electronics have shifted inside the fuselage.

I am hand launching, and notice that if I keep holding the airplane while applying full throttle, it appears that the front is being pulled down. If I lighten up the throttle it doesn't feel like the nose is being pulled down as much. This makes me think the prop needs to point up more than it is currently. I guess I'm leaning to the prop being out of alignment as the most likely cause of the issue, but it seems strange that the problem would show up immediately after a loop.

Any idea?

Thanks,
Bill
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Old Jun 16, 2012, 11:25 PM
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College Station, TX
Joined Jul 2009
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What happened, hard to say, seems you have covered the major possible issues. Be sure that the battery is not able to move about, aside from that your observation of more throttle making it head down sounds to me that the motor/prop could use some up trim. Washers under the lower mounts. Sounds like it's pointed more than 5 too far down. I mainly scratch build, so I've seen it all-

Good luck, Doug
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Old Jun 16, 2012, 11:48 PM
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Thanks Doug.

I guess we both agree that the problem is likely the motor/prop pitch. I think I'll try a new motor mount and firewall and see how that works out. The batteries I use seem to fit pretty tight and don't have a lot of wiggle room.

It is very strange that the problem appeared immediately after the loop. Currently, my theory is the G-forces of the loop loosened up the glue between the fuse and the firewall (which I noticed having a bit of give before, but thought it was safe--maybe I was wrong), which caused the motor/prop to pitch down, causing the initial wreck. Then, after my more destructive crash and gluing it all back together, the combination of a bent up motor mount with a cracked firewall still produced a pitch down condition resulting in an aircraft that still couldn't get off the ground.

It is surprising to see how the same problem reared its ugly head under two different conditions (firewall coming loose during a loop causing a prop pitch down & bent up parts also causing a pitch down), but it is certainly possible.
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Old Jun 17, 2012, 12:02 AM
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Quote:
Originally Posted by williecdog View Post
I flew my Apprentice for hours today, with no issues up until my last flight. I did a loop successfully, but then after that I could not keep the plane in the air, it just dived no matter how much elevator I gave it. It ended up crashing into some high grass. Luckily, it appeared to be intact. I checked out the plane, and then tried hand launching the same way I always do and still had the same issue: I couldn't get it to pitch up enough to get enough altitude and I ended up crashing again. I had some damage, but took care of it and tried to fly it again this evening and still the problem persisted.

I checked the COG and it appears to be close to 3.25" behind the front edge of the wing as stated in the manual. The next thing I checked was the firewall/motor mount to make sure the prop was not pointing in the wrong direction--that seemed to check out as well. I did have to glue the firewall to the fuse after the first crash, and also I glued the motor mount a bit as it was bent a little out of shape. But, despite gluing, the parts appear to be fine and the prop is pointing in the same direction as always, at least as far as I can tell.

The elevators seem to be traveling in the right direction and appear to have enough deflection to pitch the nose up, but I still can't get the plane in the air. The ailerons are working OK, they do not appear to be out of alignment. This plane has been through a lot, but everything appears to be in order. The trim appears to be OK. Both the wing and the tail appear to have the same pitch they always had. No issues with the radio (I'm using a Turnigy 9x with the RMileC model). The rubber bands seem strong enough to hold the wing to the fuse. It doesn't look like any of the servos and electronics have shifted inside the fuselage.

I am hand launching, and notice that if I keep holding the airplane while applying full throttle, it appears that the front is being pulled down. If I lighten up the throttle it doesn't feel like the nose is being pulled down as much. This makes me think the prop needs to point up more than it is currently. I guess I'm leaning to the prop being out of alignment as the most likely cause of the issue, but it seems strange that the problem would show up immediately after a loop.

Any idea?

Thanks,
Bill
You use words like 'appears', 'seems' and 'as far as I can tell'. You really need to have no doubts about these issues before flying again. For example, the CoG needs to be bang on, not just 'close'.
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Old Jun 17, 2012, 12:49 AM
Art Schmitz
United States, TN, Crossville
Joined Jan 2012
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With the wing off, hold the elevator while working the stick:
Is the pushrod flexing? Has the horizontal stab become loose? Is the control horn tight to the elevator?
Is the servo mount moving?
Is the out put arm on the servo stripped? Hold the arm, you'll feel it click.
Is the first gear on the servo output shaft stripped? You'll hear it click when holding the arm.
Just some thoughts....art
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Old Jun 17, 2012, 01:02 AM
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On my Apprentice, when new, the elevator pushrod took a pretty bad curve over the front of the plastic base of the vert fin. This made the elevator servo move a lot while working, and of course made the elev control sloppy.

I ground away plastic at the fin to remove the interference, and I just glued a piece of wood across both the rudder and elev servos to secure it. Way better.

Maybe not what happened to you, but some things to look for. Sounds to me like the thrust angle. Maybe just try to give it a toss and glide it to the ground and see if she glides straight. If it only did it under power, there you go. Good luck!
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Old Jun 17, 2012, 01:11 AM
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i might have something for you to check out on your plane. i am the head flight instructer in my club and have probably 10 apprentices on the go right now, very popular plane for a good reason. the firewall on them are glued to the foam fuse with a silicone type glue i have see it come free from the firewall on many occasions, usually freeing up on the top part of the fuse first. when the plane is at light to no thorttle the plastic peice will sit where its supposed to be but at high throttle the pull of the motor pulls it free from the firewall causing a massive negative pitch that can make it impossible to pull out of if you dont thorttle off(not the instinctive thing for a bigginer to do). rough landings put alot of stress on this glue joint and even on the ground sitting there the motor may look like its not out of place since the static forces on the firewall at that time are pushing the top into the fuse...

as for the CG comment it is NOT manditory for it to be right on the money, this plane is designed nose heavy(stable) if you move the cg back 1/2 inch you will be amazed how much nicer the plane will float in for landings and how much more fun it is to fly.

30+ years of flying and this is by far the best trainer plane i have had the pleasure of teaching people on, it has flaws but all can be ovecome easily.

todd
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Old Jun 17, 2012, 01:23 AM
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Sounds like a structural problem. How about some photos?

And is there anyone close to you that could have a look at it? RCG is great, but nothing beats hands on when trying to troubleshoot problems.

Frank
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Old Jun 17, 2012, 02:46 AM
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Quote:
Originally Posted by furballll View Post

as for the CG comment it is NOT manditory for it to be right on the money, this plane is designed nose heavy(stable) if you move the cg back 1/2 inch you will be amazed how much nicer the plane will float in for landings and how much more fun it is to fly.
Always best for lone beginners to stick to the correct CoG. So a bang on CoG IS mandatory for people like the OP when troubleshooting.
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Old Jun 17, 2012, 07:42 AM
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Originally Posted by Gerry__ View Post
Always best for lone beginners to stick to the correct CoG. So a bang on CoG IS mandatory for people like the OP when troubleshooting.
I'm sorry but this plane will tolerate a fairly wide variance in CG and still fly with little effort. In fact the plane in stock form is quite nose heavy. You can see the result by noticing almost all are trimmed with some elevator. Bang on CG usually will take a fair amount of work.
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Old Jun 17, 2012, 08:56 AM
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Originally Posted by Rich in ILM View Post
I'm sorry but this plane will tolerate a fairly wide variance in CG and still fly with little effort. In fact the plane in stock form is quite nose heavy. You can see the result by noticing almost all are trimmed with some elevator. Bang on CG usually will take a fair amount of work.
I'm not denying that, I'm saying that for a lone beginner, the recommended CoG is best, especially when troubleshooting. BTW, nailing the correct CoG should take no time at all.
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Old Jun 17, 2012, 08:59 AM
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Originally Posted by Gerry__ View Post
I'm not denying that, I'm saying that for a lone beginner, the recommended CoG is best, especially when troubleshooting. BTW, nailing the correct CoG should take no time at all.
Do you have an Apprentice? Or is this just one upping?

I guess, next, will be comparing which schools we attended?
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Old Jun 17, 2012, 09:14 AM
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Originally Posted by Rich in ILM View Post
Do you have an Apprentice?

[argumentative dross removed]
Never owned one, but set a couple up for people.
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Old Jun 17, 2012, 09:24 AM
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cdog,

For sure you DO need to acess what possibly changed, and not just go off and start changing things. That was my mistake whan I simply mentioned to change the thrust angle. Whew. So you have a good idea of what changed. It's most important to account for drastic changes in normal/expected flight with a good inspection. From all the good replies here, it's looking like the firewall, motor mount?

Art had some great T-shooting tips on that elevator. And trying a free toss w/ no throttle tells all everytime. I hope you get it back in the air better than ever, perhaps w/ some mods as outlined here.

Doug
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Old Jun 17, 2012, 10:00 AM
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Thanks for the comments, everyone!

The CoG is right where it should be. It balances just fine on the 3.25" point. Maybe it's not down to the micrometer, but it is close enough. At other times I've moved the CoG slightly by adding different equipment at different locations on the plane, and it still flew fine with a slightly shifted CoG.

I've noticed the tail structure became quite loose (mostly from side to side, not so much up and down) a few weeks back and added a little packing tape to keep it from being too loose. I also added a few pieces of some popsicle sticks underneath the stab to keep it from wiggling around so much. I've since flown with both of these mods and didn't notice any differences.

The pushrod for the elevator does flex a bit when I pull back on the stick all the way, but the elevator seems to be working OK and goes full up or full down. I've noticed the plastic guides for the pushrods don't last very long on this plane; they end up cracked.

I've tried tossing the plane with no power. It does go slightly nose down during the glide but I only throw it a few feet onto a bed as I don't want to damage it any more.

That loose firewall has been an issue a few times. I've had to glue it back on at least three times now. I have reglued it since this most recent problem showed up--it's not going anywhere at the moment.

I'll post some pics as soon as I can find that elusive camera.

Thanks again!
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